The present invention relates generally to systems for detecting and counting nuclear particles, and more particularly to counting such particles while reducing the background counting rate in gas-filled particle counters. The specific embodiments described relate to reducing background counting rates in both multi-wire counters and ionization chambers used to detect and count alpha particles, but the same techniques could be applied to counting other charged particles as well.
1. The Need for Low Background Alpha Counting
Low background alpha particle counting is important in various fields where very low concentrations of activity must be detected. Two important examples are in the testing of environmental samples and the specification of materials for the electronics industry. Data in the former case are used for such purposes as tracing radioactive emissions in the environment and estimating long term dosages to humans. In the latter case, materials which will be in intimate contact with silicon digital processing and/or storage chips must have low alpha particle emissions since these emissions can create charges within the chips that can change the values of digital numbers stored there and so introduce errors in computed or stored values.
A particular example of this is the need for low alpha lead. In presently used high density packaging technologies, silicon chips are often directly soldered to a mounting substrate using ball grid arrays or related soldering technologies. In this case the lead in the solder is in intimate contact with the silicon chip and so must have low alpha emissions for the chips to function reliably. For the next generation of high density circuits, it has been stated that:
xe2x80x9cMeasurement techniques and standards for alpha radiation effects are not adequate to support the increased alpha sensitivity anticipated for advanced technology processes.xe2x80x9d [ITRS-1999, Assembly and Packaging, pg 235]
2. Current State of the Art
There are two major techniques presently used to measure alpha particle emission: gas-filled counters and silicon spectrometers. At this point, the two have similar background counting rates, but for different reasons.
Gas-Filled Counters
To set the context of the present invention, we briefly review the operation of, and distinction between, gas-filled ionization and proportional counters, as understood by those skilled in the art. A more comprehensive presentation can be found in Knoll. [KNOLL-1989, Chapters 5 and 6]. Ionization chambers are simply gas-filled volumes fitted with electrodes so that an electric field can be applied to the volume and any charges generated therein collected. When an alpha particle traverses the gas and loses energy, it produces an ionization track, composed of gas ions and the electrons knocked off them. The more massive ions drift slowly toward the negative cathode, while the lighter electrons drift toward the positive anode about 1000 times more quickly. [KNOLL-1989, pp. 131-138]. In simple ion chambers only the total collected current is measured, which is proportional to the average rate of ion formation within the chamber. Ion chambers can also be operated as counters in pulse mode, where the currents induced in the anode by the drifting electrons are amplified and integrated so that each ionization track produces a single output pulse and is counted individually. [KNOLL-1989, pp. 149-157] However, since the induced currents flow for the full electron drift time, the amount of integrated charge produced by a track varies, depending upon its starting location within the counter. Frisch grids, whose operation is beyond the scope of this discussion, can be used to minimize this effect. In general, since detector capacitances are large and the total amounts of ionization charge produced are low, signal-to-noise is poor when ionization chambers are operated in pulse detection mode.
Proportional counters seek to increase signal-to-noise, compared to ionization chambers, by using gas avalanche gain to increase the number of charges produced. Avalanching occurs when the average amount of energy a drifting electron acquires between successive collisions with gas molecules is larger than their ionization energy. Then, on average, each collision produces a second electron and the number of electrons increases exponentially with distance. Provided the total avalanche distance is strictly limited, the final number of electrons will be strictly proportional to the starting number, but many times larger. Very large electric fields are required for avalanche multiplication to occur, of order 1 to 10xc3x97106 V/m, which are usually produced by applying a voltage of order 1 to 2 KV to a wire whose diameter is typically 0.02 to 0.08 mm in radius (0.001xe2x80x3 to 0.003xe2x80x3). Since the electric field falls of inversely proportionally to the distance from the wire""s center, avalanching can occur only within about 100 microns of the wire""s surface which, in turn, provides the limitation required to assure gain proportionality. [KNOLL-1989, pp. 160-165] Further, because essentially all the avalanche charge is produced close to the wire, there are no drifting electron induced charge effects in proportional counters, so that output pulse amplitude and charge are proportional to the initial charge in the ionization track, independent of its original location within the counter. Proportional counters are commonly operated in single pulse counting mode. [KNOLL-1989, pp. 180-185] Because the avalanche process is very fast, it lasts only as long as the ionization track arrives at the anode wire. In a well designed counter, this time is short compared to the time it takes the ions formed in the avalanche to drift away from the anode wire, typically a few microseconds. As it is this latter process that induces the detector""s output signal current in the anode, all output pulses in such well designed detectors have approximately the same shape.
The current state of the art in low background alpha counting uses a multi-wire gas-filled proportional counter with an ultra-thin entrance window. These counters can achieve sensitivities of about 0.05 xcex1/cm2/hr. [IICO-1999] They are typically constructed as shown in FIG. 1. The detector 1 includes a conducting chamber 3 sealed on one side with an ultra-thin window 4. A grid of anode wires 5 is suspended next to the chamber wall opposite the entrance window. The entire volume is filled with a counting gas 6. The anode is biased via a large value resistor 7 connected to a voltage source 8 and also connected via a capacitor 10 to a charge sensitive preamplifier 11. The preamplifier output connects to a shaping amplifier 13 and then to a discriminator 15 and counter 16. The sample 20 is placed close to the entrance window 4 and emits alpha particles into the chamber. The window 4 thus defines a sample region, namely a region of the chamber volume at or near which a sample is to be located. In other chambers, the sample may be located within the chamber, in which case the chamber structure that supports the sample would help define the sample region.
A specific alpha particle 22 is shown. This particle creates an ionization track 23 in counting gas 6. These charges drift toward the anode 5, where they are amplified by the high electric field in the vicinity of the wires and then collected. [KNOLL-1989, pp. 160-165] The resultant charge signal is integrated by the preamplifier 11, resulting in a pulse being output from the shaping amplifier 13. When discriminator 15 senses this pulse crossing a preset threshold T, it emits a short output pulse which is then counted by the counter 16.
However, in addition to ionization tracks generated by alpha particles such as alpha particle 22 emitted from the sample 20, ionization tracks 25, 26, and 27 also are generated by alpha particles emitted from the chamber backwall, sidewall, and anode wires. Because the preamplifier/amplifier pulses generated by these ionization tracks cannot be distinguished from those arising from sample-source alpha particles, these counts contribute to the detector""s background counting rate. In the current state of the art, this background counting rate is reduced significantly by constructing all of the counter""s components from materials having very low alpha emissivity. This approach not only adds significantly to the difficulty and expense of constructing such counters, but becomes exponentially more difficult as ever lower backgrounds are sought. After 20 years of development, the approach appears to have reached its natural limits.
This type of gas-filled counter has the advantage that, being filled with a low density gas, it is relatively insensitive to background radiation arising from environmentally generated gamma rays and also to most cosmic rays, which are energetic muons. These counters can also be made quite large, with commercial units up to 30 cm by 30 cm being common. Beyond its inability to distinguish alpha particle sources, the counter""s disadvantages include operational difficulties associated with the ultra-thin windows required to efficiently emit alpha particles and the sensitivity of the anode wires to microphonic pickup. Used as spectrometers, their energy resolution is poor, being 8-10% or worse.
Silicon Alpha Spectrometers
Silicon alpha spectrometers are large area Si PIN diode detectors which are biased and connected to a charge sensitive preamplifier and amplifier much as is the counter shown in FIG. 1. The major difference is that no amplification is involved: the charges generated within the Si by alpha particles are simply collected and amplified. The lack of anode wires greatly reduces microphonics and the energy required to produce a free electron in Si is about 10 times smaller than in the counting gas, so that the statistics of charge generation are much better. Energy resolutions of 1-2% can readily be obtained from such detectors. The irreducible background in these detectors is set by cosmic radiation: since the density of Si is much higher than that of counting gas, significant charge is deposited, detected and counted. With 100 xcexcm depletion depth and very careful detector design, this limit can also be reduced to about 0.05 xcex1/cm2/hr. [ORTEC-1998] These detectors are preferred when it is desirable to identify the source of the alpha particles by measuring their emitted energies.
The major advantages of silicon alpha spectrometers are their good energy resolution and relative robustness. They have two major limitations. First, it is not practical to make them in large areas, both because their capacitance becomes too large and spoils their energy resolution, and because the high quality Si required is not available in large areas. The second is the need, in low activity work, to process the sample to extract and collect all of its radioactivity (preferably with 100% efficiency) into a small source spot which can be presented to the detector. This renders these detectors impractical for measuring unprocessed or in situ samples and also adds a large overhead to measurement costs.
Related Art
The field of nuclear particle counting is highly developed, with many variations on the two counting methods described above.
The current state of the art in reducing background counting rates in gas-filled alpha counters or spectrometers is best described as xe2x80x9cpassivexe2x80x9d in that it seeks to reduce background rates solely by the method of building the counters using materials with extremely low alpha emissivities. In contrast, the present invention provides xe2x80x9cactivexe2x80x9d techniques of operating these same devices so as to achieve significant reductions in background counting rates.
The present invention employs a gas-filled alpha counter that includes a gas-filled chamber having a sample region, an anode, a preamplifier connected to the anode, and a voltage source that applies a bias such that, whenever an ionization track is generated by an alpha particle passing through the gas within the chamber, the electrons in the track are collected by the anode and cause the preamplifier to produce an output signal pulse. The output pulse is associated with the alpha particle and is characteristic of the electron collection process. Thus, both the ionization track and the resultant pulse associated with a given alpha particle can be considered to have an associated region of emanation that corresponds to the region within the chamber where the ionization track originates. A minor distinction exists between our uses of regions of emission and emanation. Region of emission refers to the place where the alpha particle departed from its source. Region of emanation refers to the place where the ionization track begins within the chamber. If the source lies within the chamber the two regions are the same. If the source is external to the chamber, as in the case of alpha particle 22, then the two regions are separated slightly.
The inventive method of operating such a gas-filled alpha counter includes, for at least some pulses, measuring one or more features of the pulse that differ depending on the pulse""s region of emanation, and determining, based on the measurement of the one or more features, the pulse""s region of emanation. Thus the counter circuitry can be considered to include a primary feature analyzer that measures the one or more features and determines information about the pulse""s region of emanation.
Thus, it is possible to discriminate between alpha particles emitted from the sample and xe2x80x9cbackgroundxe2x80x9d alpha particles emitted from other surfaces within the counter. Based on this discrimination, a pulse can be classified as background if it is determined that the associated alpha particle did not emanate from the sample region. Pulses classified as background can then be rejected, thereby effectively reducing background counting rates.
The features that can be used in performing the pulse analysis include: pulse amplitude, duration (closely correlated with collection time), slope, slope divided by amplitude, risetime, and time of arrival, used individually or in combination.
While these techniques can be applied to some existing chambers, in preferred embodiments, the invention contemplates constructing an alpha counter in a manner that exaggerates differences between preamplifier pulse features that result from collecting the ionization tracks generated by alpha particles emanating from different regions within the counter and then recognizing these differences in order to discriminate between the different regions of emanation. In this way, alpha particles from the sample can be counted, while alpha particles emitted from counter components can be identified, and possibly be rejected, resulting in a very low background counting rate, even from large counters.
Two primary approaches are employed in creating and exaggerating these pulse feature differences: first, creating different electric collection fields in different regions of the counter so electron velocities are different; and second, adjusting the counter dimensions so that charges from different regions not only take different amounts of time to be collected, but also generate different amount of induced charge in the output. In a preferred implementation, we digitize the output pulses and use digital signal processing techniques to produce the required discriminations. Using similar methods, the same discriminations can also be achieved using analog signal processing techniques.
Two specific embodiments are described to demonstrate the relevant principles. The first embodiment is a multi-wire, gas-filled counter, wherein the grid of anode wires is placed much closer to the counter backwall than to the sample wall or entrance window and is operated without gain (i.e., in ionization chamber mode) so that it is sensitive to the flow of induced charges as ionization tracks are collected. This geometric asymmetry makes the electric field in the region between the anode and the backwall much larger than the field between the anode and the sample. As they are collected, therefore, ionization tracks emanating from backwall alpha activity induce charge signals with much faster risetimes than the signals induced by ionization tracks emanating from sample wall alpha activity. Overall collection times for backwall ionization tracks are also much shorter than for sample ionization tracks, and this difference may be used as a secondary discriminator between these two sources of activity.
The second embodiment is an ionization chamber whose dimensions are adjusted so that drift lengths for collecting sample ionization tracks are much larger than drift lengths for collecting ionization tracks emitted from the backwall anode. This causes the sample track collection times to be much longer than anode track collection times, allowing them to be discriminated. Because their drift lengths are longer, sample tracks will also generate larger total induced charges, allowing signal slope, and particularly initial signal slope, divided by total induced charge to be used as a secondary discriminator in this case.
These embodiments allow reliable discriminations to be made between ionization tracks generated by the sample and ionization tracks generated by the counter backwall, which is usually its largest surface area, as well as from the anode collector wires, if any. Ionization tracks emitted from the counter sidewalls, however, are more difficult to identify by these techniques. These tracks can be reliably identified and rejected in either embodiment by the additional use of guard collectors, which are placed about the perimeter of the anode plane, parallel to it, and both close to it and close to the sidewalls as well. These guard collectors are biased at a potential close to that of the anode and connected to a second preamplifier similar to the anode""s preamplifier. Charges in ionization tracks emanating from the sidewalls are then collected on these guard collectors, producing output pulses from the attached preamplifier. The features of these pulses can then be analyzed (by a secondary feature analyzer) to identify them as emanating from the sidewalls. The simplest feature for this purpose is time of arrival: when operated in anti-coincidence with the anode, these guard collectors reliably reject sidewall source alpha emissions so that only sample source alpha particles are finally counted. Applying a further analysis of the energy in the guard collector pulses increases the efficiency with which sample source alpha particles emanating close to the edges of the sample can be reliably identified.
Applying these active methods to alpha particle counters fabricated with such common materials as lucite and copper tape allows backgrounds to be achieved that are two or more orders of magnitude lower than are obtained in state of the art counters fabricated using only passive background reduction techniques. Additional background count rate reduction can be achieved by combining these active particle source recognition techniques with the passive use of very low alpha emission counter construction materials, as in existing designs. In particular, we consider the use of highly purified, semiconductor grade silicon, although other highly purified materials are also available and could also be used effectively.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.